1. Technical Field
The present invention relates to a polarizing plate used for, for example, a liquid crystal device having liquid crystal that is driven in a VA (Vertical Aligned) mode, and the liquid crystal device having such a polarizing plate. In addition, the invention further relates to an electronic apparatus including, but not limited to, a liquid crystal projector or a direct-view type display device that is provided with such a liquid crystal device.
2. Related Art
A liquid crystal device of the above-described type nas, typically, a polarizing plate on either surface of its liquid crystal panel. Such a polarizing plate is configured to include a polarizing element, which has a polarizing film made of polyvinyl alcohol (PVA), etc., that defines polarization of light and protection layers each of which is deposited on either surface of the polarizing film and is made of cellulose triacetate (TAC), etc., and to further include a supporting substrate that supports these components. One polarizing plate that is provided at the incoming-light side (i.e., light-incident side) of the liquid crystal panel selectively transmits, among all incident light that enters the incoming-light-side polarizing plate, linearly polarized light that has an amplitude component parallel to the transmission axis of its polarizing film, and then emits the transmitted linearly polarized light to the liquid crystal panel. The other polarizing plate that is provided at the outgoing-light side (i.e., light-emitting side) of the liquid crystal panel selectively emits, among all emission light that goes out from the liquid crystal panel, linearly polarized light that has an amplitude component in a predetermined direction at a display region of the liquid crystal device.
In a related-art liquid crystal device having such a configuration, it is frequent that the transmission axis of the polarizing film and the optical axis of the protection layer get shifted from each other either at the time of production of such a polarizing plate or due to poor axial alignment precision thereof. Since the contrast of the liquid crystal device decreases when an axial shift occurs, as a conventional solution proposed by, among other publications, Japanese Patent No. 3327410, each of the protection layers such as TAC layers is configured to be thin such that the protection layer has a low retardation value, thereby reducing or mitigating any impact of the optical-axis shift of the protection layer on the contrast performance of the liquid display device.
Disadvantageously, however, a polarizing plate having protection layers that are made of organic material such as TAC, the protection layer itself could become distorted due to external factors, for example, heat, mechanical stress, etc. Such a distortion would in turn form a plurality of optical axes extending in directions varying from one to another in the distorted protection layer. These plurality of optical axes change the birefringence in each region of the distorted protection layer, which results in the deteriorated contrast of the liquid crystal device having the affected polarizing plate.
The above problem of related art is further described in detail as follows. In a theoretical sense, if the transmission factor, or in other words, transmissivity, of the liquid crystal device is measured under the “cross nicol” arrangement of two polarizing plates where the transmission axis of one of the polarizing plates are arranged to be orthogonal to that of the other polarizing plate, the transmission factor would be zero percent when the angle of the transmission axis of each of the polarizing films is aligned with the angle of the optical axis, such as a phase-delay axis or a phase-advance axis, of the corresponding protection layer. However, even a slightest shift between the transmission axis of the polarizing film and the optical axis of the protection layer raises the transmission factor. That is, due to the distortion of TAC functioning as the protection layers, optical axes varying in angle thereof from one to another would be formed in the TAC, which results in a disadvantageously heightened transmission factor in the cross nicol arrangement. If such an undesirable polarizing plate is built in a liquid crystal device, the transmission factor of the liquid crystal device in a black display operation state will become higher so as to decrease contrast thereof. Since the decrease in contrast is attributable to the distortion of the protection layer, the phenomenon of contrast decrease become more perceivable as the film thickness of the protection layer such as SAC is made thinner. Moreover, such a phenomenon of contrast decrease is more perceivable in a vertical-alignment display scheme (i.e., VA mode) where images are displayed under a “normally-black” mode.